Marge Simpson in 'Screaming Yellow Honkers' - S10-E15
Superintendent Chalmers: Well Seymour, it seems we've put together a baseball team, and I was wondering who's on first.
Principal Skinner: Yes, not the pronoun, rather a player with the unlikely name of "Who" is on first.
Superintendent Chalmers: Well that's just great Seymour, we've been out here six seconds, you've already managed to blow the routine!
Answer: I'm not sure if this is the one you're thinking of, but an episode of "Ray Bradbury Theatre," called "A Sound of Thunder," dealt with a similar matter: a group of hunters travel back in time to hunt dinosaurs, only to find things have changed when they get back because someone stepped on a butterfly.
Xofer